Hello

Thanks for using Just Answer.

Please do a system restore will make your system as earlier..

1. Click Start, point to All Programs, point to Accessories, point to System Tools, and then click System Restore. System Restore starts.
2. On the Welcome to System Restore page, click Restore my computer to an earlier time (if it is not already selected), and then click Next.
3. On the Select a Restore Point page, click the most recent system restore point in the On this list, click a restore point list, and then click Next.
   
   Note Select the date in which firefox is working fine. Click OK.
4. On the Confirm Restore Point Selection page, click Next. System Restore restores the previous Windows XP configuration, and then restarts the computer.
5. Log on to the computer as Administrator. The System Restore Restoration Complete page is displayed.
6. Click OK.

Hello

Check these articels

LINK1

LINK2

First article describes where is your mozilla profile will be..

In that you have to copy places.sqlite file ..This file contains the book marks..

And second article describes creating new profile..

After creating new profile..Copy that file in to that profile..

Let me know any doubts..

Atleast the backup your mozilla firefox ..

Please follow the steps in to backup the profile ..

this is for windows XP...

1. Click the Windows Start button, and select Run....
   
   
   
2. Type in %APPDATA%\Mozilla\Firefox\Profiles\ then click OK.
   
   
3. Windows Explorer opens a folder containing your profiles.
4. Double-click the profile folder you wish to open (e.g. the xxxxxxxx.default folder).